Generally, in an OFDM transmitting/receiving apparatus, a frame configuration is used in which a signal with the same waveform as the last part of effective symbols is added as a guard interval (hereinafter referred to as “GI”) at the start of effective symbols. A delayed waveform with a delay time shorter than the guard interval length can be eliminated by Fast Fourier Transformation (hereinafter referred to as “FFT”) processing in the receiving system. On the other hand, if the multipath delay time is longer than the GI length, or if there is timing error, the preceding signal may leak into the effective symbols of the next signal, and inter-code interference may occur.
In the transmitting system, a signal that has undergone Inverse Fast Fourier Transform (hereinafter referred to as “IFFT”) processing has a GI inserted and is converted from a digital signal to an analog signal, and a transmit signal is obtained.
In the receiving system, a received signal is converted from an analog signal to a digital signal. Then the received signal from which GIs have been eliminated by a GI elimination circuit undergoes FFT processing, and a baseband signal is obtained. The baseband signal undergoes coherent detection by means of a coherent detector, and a coherent detected signal is obtained.
Nowadays, in radio communications, and especially in mobile communications, various kinds of information such as images and data are transmitted as well as voice. Henceforth, demand for the transmission of various kinds of content is expected to continue to grow, further increasing the necessity of highly reliable, high-speed transmission. However, when high-speed transmission is carried out in mobile communications, the effect of delayed waves due to multipath propagation can no longer be ignored, and transmission characteristics degrade due to frequency selective fading.
Multicarrier (MC) modulation methods such as OFDM (Orthogonal Frequency Division Multiplexing) are attracting attention as one kind of technology for combating frequency selective fading. A multicarrier modulation method is a technology for effectively performing high-speed transmission by transmitting data using a plurality of carrier waves (subcarriers) whose speed is suppressed to a level at which frequency selective fading does not occur. With the OFDM method, in particular, the subcarriers to which data is allocated are mutually orthogonal, making this the multicarrier modulation method offering the highest spectral efficiency. Moreover, the OFDM method can be implemented with a comparatively simple hardware configuration. For these reasons, the OFDM method has attracted particular attention and is the subject of various studies.
Conventionally, received signal transmission error detection is performed, and when an error is detected, a retransmission request signal is sent to the communicating radio station. On receiving a retransmission request, the communicating radio station retransmits data corresponding to the retransmission request. This processing is then repeated until there is no longer an error in the received signal. This series of processes is called ARQ.
However, with a conventional transmitting apparatus and transmitting method, particularly when channel fluctuations are slow, errors may occur consecutively even when retransmission is performed to a specific user requesting retransmission. In this case the number of retransmissions increases excessively, and since propagation delay increases as the number of retransmissions increases, there is a problem of increased transmission delay. There is a method for preventing this increase in transmission delay by discontinuing retransmissions at a given delay time, but in this case, there is a problem of error rate degradation. Furthermore, since new data is not contained in a GI, there is also a problem of transmission efficiency decreasing as a GI is made longer.